An F-Stop of 2.9 on a camera lens means that the aperture is wide open reducing the amount of DoF.  As your F-Stop value increases, the size of the aperture decreases thus increasing the amount of DoF in the scene.  This is an oversimplification of how Poser handles focus distance and DoF, there are other factors like ISO and lighting conditions that govern how F-Stops are determined on a camera lens, depending on the lens.